This invention relates in general to a system for measuring radiance of radiation sources, and in particular, to luminance meters of improved performance.
Luminance meters have wide applications. It is used, for example, in radiology for calibrating the intensity of radiation sources such as x-ray sources in order to be able to compare images for improved accuracy.
Conventional luminance meters employed circular apertures for passing radiation from the radiation source to the sensor. In such conventional luminance meters, due to the symmetry of the circular apertures employed, stray radiation has the maximum probability of reflection at the edges of the apertures to reach the sensor.
When a conventional luminance meter is employed to measure radiation sources with rapidly varying intensities, depending on the sampling time period chosen, the meter may give a reading that is widely different from that perceived by the human eye. For example, when the conventional luminance meter is employed to measure the radiance from a cathode array tube screen, for example, the radiance from the phosphor decays rapidly. Therefore, depending upon the sampling time period of the conventional luminance meter employed when the screen is measured, the radiance measured may be that from the phosphor after the intensity of the radiance has declined significantly, so that the reading given by the meter may differ from that observed by the human eye by a significant amount.
While conventional luminance meters may be adequate for some applications, they do not provide adequate dynamic range and resolution for other applications such as in radiology requiring higher performance. It is, therefore, desirable to provide improved luminance meters that can meet the needs of such other applications with higher performance requirements.
This invention is based on the observation that by employing an asymmetric aperture or a combination of asymmetric apertures, the amount of stray radiation received by the sensor or detector can be reduced compared to that in the case of the conventional luminance meters employing circular apertures. This invention is also based on the observation that, by filtering radiation from the source to be measured by a filter having a temporal frequency response that mimics that of the human eye, the accuracy of measurement can be much improved. In addition, performance of the meter can be enhanced by integrating the output of the sensor and sampling the integrated output at time intervals that are exponential functions of time to provide a reading. In this manner, high and substantially constant resolution of the meter can be achieved when the meter is used to measure radiation sources of very different intensities.